Synthesis of miktoarm start polymers via controlled polymerization systems

Abstract

Complex macromolecular structures such as star polymers have been synthesized in the search for polymers with improved mechanical and thermal properties. Star polymers are branched polymers consisting of several linear chains linked to a central core. Among all branched structures, star polymers have been certainly the most investigated architectures, attracting much experimental and theoretical interest. Such species have been very useful in providing further insight into how branching affects the overall properties of polymers in solution or in melt. Some of the applications involving star polymers are the direct result of these structureproperty relationships, these polymers being now commonly used as viscosity modifiers in paints and coatings or for their improved processability and mechanical properties compared to their linear analogues. Star polymers containing chemically different arms are termed miktoarm or heteroarm star polymers. Miktoarm is the combination of Greek word miktos meaning ˝mixed˝, and “arm”. Compared with the corresponding linear block copolymers, miktoarm star polymers exhibit many interesting properties, such as unique phase separation behavior either in bulk or in solution, due to steric hindrance as a result of more than two different types of polymers being brought together at a single junction (core). Although star polymers constitute the simplest branched structure, their synthesis remains challenging, and star polymers are often difficult to synthesize in a well-controlled manner. Due to the complex nature of these macromolecules, living polymerization techniques, such as anionic, cationic have typically been used to obtain well-defined starshaped macromolecules. The early synthesis of miktoarm star polymers have been based on two general strategies. The first involves living anionic polymers being consecutively reacted with an appropriate multifunctional core (chlorosilane compound) in a consecutive polymer reaction. The second is the reaction of the active chain with divinylbenzene (DVB). Living polymerization is a chain growth polymerization that proceeds in the absence of irreversible chain transfer and chain termination. Living polymerizations provide the maximum degree of control for synthesis of polymers with predictable, well-defined structures. For a long period of time, living ionic polymerization (anionic or cationic) was the dominant living polymerization method. However, in recent years there has been rapid growth in the area of growing controlled/“ living” radical polymerizations (CRP), which have some advantages over anionic polymerization, in that they do not require rigorous experimental conditions. CRP is a simple and robust method for the synthesis of complex macromolecular structures with low polydispersity and well-controlled architecture and functionality. Atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMP) are the most widely used CRP methods. In addition, controlled ring-opening polymerization (ROP) has found wide applications in the polymerization of lactones and lactides. This study focused on the synthesis of well-defined miktoarm star polymers based on combination of controlled radical and nonradical polymerization systems by using a core-first approach employing miktofunctional initiators. An ABC-type miktoarm star polymer was prepared with a core-out method via a combination of ringopening polymerization (ROP), stable free-radical polymerization (SFRP), and atom transfer radical polymerization (ATRP). First, ROP of ε- caprolactone was carried out with a miktofunctional initiator, 2-(2-bromo-2-methyl-propionyloxymethyl)- 3-hydroxy-2-methyl-propionic acid 2-phenyl-2- (2,2,6,6-tetramethylpiperidin- 1-yl oxy)-ethyl ester. Second, previously obtained poly($varepsilon$-caprolactone) (PCL) was used as a macroinitiator for SFRP of styrene. As a third step, this PCL–polystyrene (PSt) precursor with a bromine functionality in the core was used as a macroinitiator for ATRP of tert-butyl acrylate This produced an ABC-type miktoarm star polymer [PCL–PS–poly(tert-butyl acrylate)] with a controlled molecular weight and a moderate polydispersity. Furthermore, one-pot synthesis of PCL-PS precursor was carried out via combination of ROP-NMP routes. Finally, the subsequent ATRP of methyl methacrylate (MMA) using PCL-PS precursor as macroinitiator gives PCL-PS-PMMA miktoarm star polymer. The obtained polymers were characterized with gel permeation chromatography and $^1H NMR$

Özet

Yıldız polimerler birkaç lineer polimer zincirinin bir merkez çekirdeğe bağlı olduğu dallanmış yapılardır. Farklı kollu yıldız polimerler ise sahip oldukları farklı molekül ağırlığı ve kimyasal kompozisyonda kollardan dolayı oldukça ilgi uyandırmaktadır. Kontrollü polimerizasyon sistemleri iyi tanımlanmış kompleks makromoleküler yapıların sentezi için oldukça etkili bir yöntemdir. Bu çalışma, kontrollü polimerizasyon yöntemleri kullanılarak iyi tanımlanmış yapıya sahip farklı kollu yıldız polimerlerin sentezi üzerine yoğunlaşmıştır. Bu amaçla, atom transfer radikal polimerizasyonu (ATRP), kararlı serbest radikal polimerizasyonu (SFRP) ve halka açılma polimerizasyonu (ROP) için uygun fonksiyonel gruba sahip yeni bir başlatıcı, 2-(2-bromo-2-metilpropiyoniloksimetil)- 3-hidroksil-2-metil-propiyonikasit 2-fenil-2-(2,2,6,6 tetrametilpiperidinil oksi)- etil ester, sentezlendi. ABC tipli farklı kollu yıldız polimerin eldesi için iki farklı yol izlendi. Birinci yaklaşımda, sentezlenen başlatıcı kalay oktoatın $(Sn(Oct)_2)$ katalizör olduğu $varepsilon$-kaprolaktonun (KL) halka açılma polimerizasyonunda kullanılarak poli($varepsilon$-kaprolakton)(PKL) makrobaşlatıcısı elde edildi. Sentezlenen PKL stirenin (St) SFRP’sinde makrobaşlatıcı olarak kullanıldı ve polistiren (PS)-blok-PKL blok kopolimeri sentezlendi. Son olarak, uç grubunda ATRP fonksiyonel grubuna sahip PS-blok-PKL blok kopolimer, tersiyer-butilakrilatın (tBA) ATRP’sinde makrobaşlatıcı olarak kullanıldı ve nihayetinde PKL, PS ve poli(ter-butilakrilat) (PtBA) kollarına sahip, düşük molekül ağırlığı dağılımlı ABC tipli farklı kollu yıldız polimer elde edildi. İkinci yaklaşımda ise, ROP ve SFRP yöntemleri aynı anda kullanılarak tek aşamada PKL-blok-PS blok kopolimeri sentezlendi ve metilmetakrilatın (MMA) ATRP’sinde makrobaşlatıcı olarak kullanıldı sonuç olarak PKL, PS ve poli(metil metakrilat) (PMMA) kollarına sahip ABC tipli farklı kollu yıldız polimer elde edildi. Sentezlenen yıldız polimerler $^1H-NMR$, GPC ölçümleriyle analiz edildi.